The present invention relates to a method and an apparatus for the desalination of seawater.
Fresh water and drinking water are a valuable raw material which is in urgent need not only in particularly dry climatic zones, but also in industrialized countries where the demand is increasing more and more. The growing scarcity of existing resources is leading to substantial efforts to find new sources of drinking water and to treat non-potable water, for example seawater, by cleansing. Especially in very dry regions, the processing of water suitable for plant irrigation from seawater is a necessity.
Usual methods of desalinating seawater are reverse osmosis (RO), vapour recompression evaporation (VR) and multiple-effect evaporation (MSF, ME). In reverse osmosis, the seawater is forced through a semipermeable membrane and cleaned in the process. However, the disadvantage of such RO plants is their very high sensitivity to polluted, especially oil-polluted water, so that it is necessary to constantly control the incoming water. In the VR method, which does not work at excess pressure, the seawater is evaporated under reduced pressure and then condensed again by compression. Evaporation at subatmospheric pressure means that drinking water of good quality can only be obtained from seawater, in coastal regions which is possibly highly contaminated bacteriologically and also further organically contaminated, at additional high, especially chemical expenditure. This is so for the following reasons: firstly, at such a low evaporation temperature, practically only the non-volatile components of the water can be separated from it, for example the salt from seawater, which can be extracted by phase transformation alone. In most cases, separation of the volatile components from the water is not possible by this method alone. Secondly, the disadvantage of the vacuum evaporation method becomes even clearer on observing the microbiological side of distillation. From published investigations, it is known that bacteria, germs and the like can only be killed at a certain temperature and after a certain period as a function of that temperature. Depending on the level of contamination, this is an absolutely essential aspect in the processing of seawater into drinking water, because the method does not ensure that non-evaporated water or particles from same will not also be transferred to the distillate. Thirdly, vacuum evaporation methods of this kind require a pump in the distillate pipe, which extracts the distillate produced from the apparatus under vacuum, this being the only manner in which the pressure difference between the interior under vacuum and normal pressure can be overcome. The result is none other than renewed contamination of the distillate by the moving parts of the pump and increased expenditure in terms of apparatus and maintenance. Multiple-effect evaporation, which, as an energy source, requires thermal energy in the form of hot vapour, disadvantageously involves the continuous consumption of treated water which is used to generate the hot vapour required for the evaporation process. In very dry regions, such as deserts, this water is not available, or reduces the distillation capacity of the plant because it must be drawn from same.
Care must be taken in connection with the desalination of seawater that the solubility limit of CaSO.sub.4 (calcium sulphate) is not exceeded. Saturation is reached more rapidly, however, as concentration and temperature increases, so that the solubility limit is reached at about 110.degree. C. and the standard seawater anhydrite calcium-sulphate concentration. In seawater with higher salt concentrations, this limit is reduced to lower temperatures (H. E. Honig, "Seawater and Seawater Distillation", pages 68 and 69, Fichtner Handbook, published by Vulkan, Essen). The more the process of seawater desalination enters an area of high salt-content in the course of evaporation, the greater will be the level of precipitation of the three calcium-sulphate modifications. It is essential to ensure that, in each of the aforementioned processes, precipitation does not occur in such a way as to block membranes or clog evaporators. Corresponding concentrations can be avoided if wash-out rates are high. Usually, to avoid sedimentation, the seawater is dosed with additives which prevent crystallisation of the modifications occurring in calcium sulphate, for example (anhydride, water-containing calcium sulphate and gypsum), or produce "threshold" effects in the presence of hardeners. The disadvantage in operating with high wash-out rates is an additional consumption of energy, both in RO plants as a result of higher pumping capacity and in MSF and VR plants on account of heating the required quantity of seawater which is now greater by the quantity of wash-out water and must be heated to the respective boiling temperature.
While a method and an apparatus for distillation of untreated water have become known from DE 26 00 398 C2, respectively in and by which previously desalinated and demineralised untreated water is treated and processed into a high-purity, germ-free distillate, approximately the same quantity of distillate being produced as untreated water supplied to the apparatus, this method and this apparatus for distillation of untreated water are not suitable in this form for the desalination of seawater. If a corresponding quantity of seawater were fed into this known apparatus, the primary system would become clogged in a very short space of time with, for example, all three calcium-sulphate modifications and other salts originating from the seawater.